The monitoring and control of the air traffic which is constantly increasing worldwide is currently based primarily on two fundamental technologies: radar monitoring and voice-based VHF radio systems for Air Traffic Management (ATM). In the case of radar monitoring, radio signals are transmitted using special radar ground stations, said radio signals then either being reflected by the flying object on its outer surface and received once more by the ground station (primary radar) or causing the flying object autonomously to send back specific information to the ground station (secondary radar). However, in both cases it is necessary for the flying object to be located within a coverage area dependent on the range of the radar ground station, as no usable signals would otherwise be produced. Since radar ground stations of this type are very expensive to set up and are therefore also usually found only in regions with dense traffic or near the coast, gapless radar monitoring of flying objects is not currently provided.
If a flying object is located outside the radar coverage, the corresponding monitoring and control stations responsible for the air-traffic control of the corresponding flying object maintain contact at regular intervals via the VHF radio system. However, the VHF radio system is based entirely on the principle of voice messages, so that it often occurs in the case of long-haul flights that the corresponding monitoring and control station receives no information from the aircraft over a lengthy period because, for example, the autopilot is activated and the pilots are asleep. The status of the flying object is therefore uncertain over a lengthy period and is not therefore suitable for a continuous gapless monitoring.
ADS (“Automatic Dependent Surveillance”) systems are known in addition to the primary and secondary radar already mentioned. In ADS systems, corresponding data are automatically and continuously collected on board the aircraft, including the current position, which can be determined, for example, using a satellite navigation system. Other flight data may, for example, be the flight number, aircraft type, speed, flight altitude and planned flight direction. If a corresponding radio signal transmitted from a ground station is received by the flying object, the flying object transmits these flight data using a corresponding ADS message to the ground station. The ground station receives the message and can display the flight data contained in the message to the air-traffic controllers or control stations. These systems, which transmit the messages on request, are also frequently referred to as ADS-C or ADS-A. For this purpose, the aircraft are equipped with so-called FANS-1/A devices, which enable a radio data link via VHF, RF or satellite radio and therefore also a monitoring of the aircraft in oceanic airspace. However, the position messages are transmitted at time intervals of around 15 minutes or more due to the low data rate, so that a continuous flight monitoring is not provided.
Along with the systems which transmit their flight data entirely on request, the ADS-B (Broadcast) system also exists, which autonomously transmits a corresponding ADS message at periodic, discrete-time intervals of around half a second. If the flying object is located in a region with dense traffic, an entire range of ground stations is normally located here, which can receive the automatically transmitted ADS message. In addition, it is also possible for other flying objects which have a corresponding receiver to be able to receive ADS-B messages from other aircraft, in order thus to build up a picture of the traffic situation of the surrounding airspace depending on the flight data contained in the message.
Since the equipment of transport aircraft with mode-S transponders is currently prescribed as mandatory, the mode-S 1090ES data transmission system has become established as the de facto standard for the ADS-B system in general aviation. Mode-S 1090ES is based on the modulation methods and data formats defined for mode-S, wherein a fixed, worldwide unique 24-bit address is allocated to each aircraft. The downlink in mode-S is established at a frequency of 1090 MHz.
However, as with all systems based on radio location, the ADS systems have the decisive disadvantage that the aircraft must be located in the receiving area of a corresponding receiving unit so that the ADS messages transmitted by the aircraft can also be received by the corresponding supervisory control station. Due to the limited range of the ADS-B signals, a worldwide gapless monitoring using ground stations is not possible even with a system of this type since, on the one hand for cost reasons, and, on the other hand due to geographical conditions, ground stations cannot be set up all over the world for gapless coverage.
In order to guarantee a reception of ADS signals even in areas in which no coverage is guaranteed by corresponding ground stations, DE 10 2008 026 415 A1 and DE 10 2008 013 357 A1, for example, propose that the ADS-B signals transmitted by a flying object can be received by a satellite correspondingly suited for this purpose. The messages can then be forwarded from there to a ground station or can be transmitted to a relay satellite in order to achieve a greater transmission path. Due to the fact that a satellite monitors the ADS-B signals regularly transmitted by the aircraft, a partial monitoring can be carried out precisely in areas where no coverage with ground stations is possible.
However it is disadvantageous here that the use of relay satellites for range extension is very costly, since the operators of relay satellites of this type levy a charge for the use thereof. In addition, a ground station must be able to communicate with a multiplicity of different satellite types in order to be able to receive corresponding ADS messages in practice also.